The power industry and the fast developing high speed railway systems need handy tools for high voltage transmission line service, fault location and repair. A mongst various tools, a portable current and voltage monitor is the most desirable. Instruments similar to clamp current meters would find wide applications, because they don't need a direct ground connection and there is not need to disconnect or cut the power liner. In order to work on live power lines, these instruments must be able to withstand high voltage, normally, up to 1 million volts.
In the past few decades, fiber optic current transformers and voltage transformers have attracted much attention, due to their natural suitability for working in HV environment. Such transformers have already been employed in power stations.
Commercial fiber optic current/voltage transformers are mainly designed for permanent installation, not for mobile applications. This is because these transformer are bulky and because the power lines need disconnection during these transformers' installation.
Some types of portable fiber optic current/voltage transformers are proposed, and there are commercial vehicle borne optical current or voltage sensors for applications on up to 500 kV power lines (1, 2). These instruments are highly accurate and they are suitable for on site calibration purposes. However, they are bulky and expensive, and it is not so easy and convenient to wrap the sensing fiber head on an energized conductor. Therefore, their mobility is limited and they are not suitable for maintenance and repair work. A handheld instrument is more desirable than the vehicle borne transformers.
Traditionally, electric current flowing in a power line can be measured in one of the following three ways without disconnecting the conductor:    1. Using a split iron core current transformer. Clamp Ammeters use this method;    2. Using a split (air core) Rogowski coil and an integrator. This method avoids magnetic saturation suffered by iron core current transformer;    3. Measuring the voltage drop across a certain length of the conductor. When the contact points move along the conductor, the relative change of measured voltage reflects the resistance of the conductor and thus wire damage can be located.
The above mentioned methods can be combined with fiber optic technology to make fiber optic current sensors. Because both electric and optic means are used, this type of fiber optic current sensor is called hybrid type.
A prior art (3) proposed an AC current sensor based on Diffractive Microelectromechanical systems (DMEMS) device. The DMEMS device is a variable optical attenuator, which attenuates the input optical signal by a control electric signal. In the proposed current sensor, the DMEMS device modulates the input constant optical signal according to an AC voltage which is generated by a current transformer or an air core Rogowski coil. The DMEMS device has a very high electric impedance, and therefore it consumes negligible electric power of micro Watts. A DC bias of 5-6 volts and an AC drive voltage of no more than 1 volt peak to peak are adequate for the sensor to work. One of the major advantages of this type of optical current sensor is that it requires very little electrical power. Therefore power supply for HV side circuitry is greatly simplified compared conventional hybrid fiber optic current sensors. A battery of one Amp hour capacity will keep the sensor working for many years. Another prior art (4) described an electric field sensor also based on the DMEMS device. The electric field sensor head includes a condenser antenna, a DC bias, and a DMEMS device. The condenser antenna picks up electric field and turns it into a voltage. This voltage then drives the DC biased DMEMS device, and an optical receiver recovers the voltage that is applied to the DMEMS device which represents the electric field strength.
This invention describes a handheld fiber optic current and voltage monitor for applications in HV environment.
The current sensor is DMEMS device based. It includes
The voltage sensor is based on an electric field sensor which employs a DMEMS device in the sensor head.
When the electric field strength near a power line is measured, the voltage on the power line can be calculated.
Every energized conductor generates an electric field around it. For a round conductor with infinite length, the electric field is:
      E    =          q      r        ,where q is the electric charge per unit length, and r is the distance from the conductor center.
A convenient way to measure the electric field of a energized power line is to place the measuring sensor in close proximity to the line. However, power line conductors have various diameters. Therefore, the measured electric field strength will have vastly different values when the line voltage and the light from ground are the same but the conductor has a different thickness. For example, electric field measured on a 30 mm thick conductor will be only ⅔ that measured on a 20 mm thick wire given that other parameters are identical. This would lead to intolerable errors if the line voltage is calculated from the measured electric field.
This invention proposes a simple method to overcome this error. A split metal cylinder is clamped on to the conductor wire, and thus the cylinder has an identical electric potential as the conductor wire. The electric field is measured near the surface of the cylinder. Therefore, no matter what the wire's diameter is, the wire will have an effective diameter of the cylinder. Errors caused by different wire diameters are thus eliminated.